The present invention relates to a method for determining the efficiency of an exhaust gas purification device of an internal combustion engine, in particular of a diesel internal combustion engine for motor vehicles, and to an apparatus for determining the efficiency of an exhaust gas purification device of an internal combustion engine, in particular a diesel internal combustion engine for motor vehicles.
Exhaust gas purification devices of the type in question for diesel internal combustion engines, in particular in motor vehicles use two NOx sensors for system control and monitoring, one of which is arranged upstream of an oxidation catalyst and the second is arranged downstream of a reduction catalyst and the signal values of which allow conclusions to be drawn on the NO, NO2 and NH3 concentrations in the exhaust gas.
It is known that the conversion rate of oxidation catalysts and/or of particulate filters can decrease due to ageing or, where relevant, due to sulphur contamination (in the case of unsuitable fuels), wherein monitoring of this decrease would be advantageous for the purpose of specifying component replacement or for the adaptation of control unit characteristic values, curves or maps in an electronic engine control unit.
DE 10 2011 101 174 A1 discloses arranging a first NOx sensor upstream of an oxidation catalyst, a second NOx sensor downstream of a particulate filter arranged after the oxidation catalyst, and a third NOx sensor downstream of an SCR catalyst arranged after the particulate filter. The first and second NOx sensors, which are each arranged upstream of the SCR catalyst, have an NO2 cross sensitivity in order to determine the NO/NO2 concentration in the exhaust gas without being impaired by the downstream supply of reducing agent. By comparing the signal values of the two NOx sensors in an electronic control unit, it is possible to infer a reduced conversion rate of the oxidation catalyst and/or of the particulate filter and, if appropriate, to generate a fault signal. In contrast, the third NOx sensor, which is arranged downstream of the SCR catalyst, should have only an NH3 cross sensitivity in order to be able to detect a reducing agent breakthrough. The outlay on components is relatively high here, as is the outlay on evaluation.
WO 2010/097292 A1 describes another method for estimating the effectiveness of an oxidation catalyst, which manages without measuring probes or NOx sensors on the oxidation catalyst and, for this purpose, measures the effectiveness or temperature spectrum of the reduction catalyst by means of a temperature sensor and evaluates it in a characteristic-curve memory. The temperature spectrum in the SCR catalyst may shift upwards as the conversion rate of the oxidation catalyst decreases, for example.